1. Field of the Invention
The present invention relates to a method for manufacturing a self-alignment type bipolar transistor having an epitaxial base layer.
2. Description of the Related Art
Generally, in a bipolar transistor, the thinner a base layer, the larger the operation speed of the bipolar transistor. That is, when the base layer is thinner, the carrier transit distance is decreased to increase the operation speed.
A first approach is to implant impurity ions via a thin silicon oxide layer into a base forming region of a semiconductor substrate at a low acceleration energy. Then, the impurity ions are thermally diffused into the base forming region. In this case, however, it is difficult to make a shallower base region due to the channelling phenomenon.
A second approach is to form a boron-silicated glass (BSG) layer on a base forming region of a semiconductor substrate and thermally inject boron ions into the base forming region. In this case, however, since the degree of diffusion within the BSG layer is different from that within the semiconductor substrate, it is difficult to obtain a stable impurity profile within the base forming region.
A third approach is to grow an epitaxial base layer including impurities on a semiconductor substrate. In this case, an impurity profile within the epitaxial base layer can be precisely determined by programming an epitaxial process, so that the epitaxial base layer, i.e., the base region can be thin.
On the other hand, in view of the reduction of a parasitic capacitance and a parasitic resistance of a bipolar transistor, a self-alignment bipolar transistor has been developed to increase the operation speed.
A first prior art method for manufacturing a self-alignment type bipolar transistor having an epitaxial base layer is disclosed in JP-A-4-56328. That is, after a polycrystalline silicon layer as a base contact (electrode take-out) is formed at the periphery of a base forming region of a semiconductor substrate, an epitaxial growth process is carried out, so that an epitaxial base layer is grown from the semiconductor substrate and a polycrystalline silicon layer is grown from the above-mentioned polycrystalline silicon layer. This will be explained later in detail.
In the first prior art method as illustrated, however, an emitter-base (PN) junction made of polycrystalline silicon is created, which generates a recombination current.
In addition, since the surface of the semiconductor substrate is damaged by an anisotropic etching process, crystal defects are created in the semiconductor substrate. As a result, crystal defects are induced in the epitaxial base layer, which easily create a recombination current.
A second prior art method for manufacturing a self-alignment type bipolar transistor having an epitaxial base layer is disclosed in JP-A-7-183310. That is, after a polycrystalline silicon layer as a base contact (electrode take-out) is formed on a silicon oxide layer at the periphery of a base forming region of a semiconductor substrate, an epitaxial growth process is carried out, so that an epitaxial base layer is grown from the semiconductor substrate and a polycrystalline silicon layer is grown from the bottom of the above-mentioned polycrystalline silicon layer. Then, the grown polycrystalline silicon layer is covered by a sidewall BSG layer. This will be explained later in detail.
In the second prior art method, however, since the BSG layer is formed by a chemical vapor deposition (CVD) process, the surface of the epitaxial base layer is contaminated a surface level is easily generated in the epitaxial base layer. This creates a recombination current. In addition since the surface of the epitaxial base layer is damaged by the anisotropic etching process for forming the sidewall BSG layer, crystal defects are created in the epitaxial base layer. As a results crystal defects are also induced in an epitaxial emitter layer. Such crystal defects easily create a recombination current.
Note that the recombination current decreases the current gain of the bipolar transistor.